The present invention relates to a method for making aromatic organic carbonates such as diphenyl carbonate by effecting reaction between an aromatic organic hydroxy compound, such as phenol, and carbon monoxide and oxygen in the presence of an effective amount of a palladium carbonylation catalyst. More particularly, the present invention relates to the carbonylation of an aromatic organic hydroxy compound utilizing an inorganic cocatalyst, such as a cobalt complex containing a pentadentate Schiff base in combination with the palladium catalyst.
Procedures for making diorganic carbonates are shown by Hallgren, U.S. Pat. Nos. 4,361,519 and 4,410,464, utilizing a molecular sieve as a drying agent for the water formed during the reaction. An additional carbonylation method for making diorganic carbonates is shown by Japanese patent 01,165,551. Aromatic organic carbonates are of particular interest to thermoplastic manufacturers, since they offer an alternative non-phosgene route to aromatic polycarbonates by melt transesterification. A procedure for making aromatic organic carbonates using an organic solvent, such as methylene chloride, is shown by Chalk, U.S. Pat. No. 4,187,242. Reference also is made to T. C. Chang in copending application Ser. No. 217,248, filed Jul. 11, 1988, and EP350-700-A, utilizing a divalent or trivalent manganese salt or cobalt (II) salt and hydroquinone in combination with a palladium catalyst, to catalyze the conversion of an aromatic organic hydroxy compound, such as phenol, to an aromatic organic carbonate. U.S. Pat. No. 4,218,391, Romano et al employs a copper salt to prepare organic esters of carbonic acid. Attempts to use such catalyst with aromatic organic hydroxy compounds, such as phenol, under constant flow conditions have been found to provide unsatisfactory results with respect to % carbonate yields and % carbonate selectivity as compared to the use of aliphatic hydroxy compounds, such as methanol, in preparing aliphatic carbonates under substantially the same conditions.
In application EP350-700-A and copending application Ser. No. 07/906,681, carbonylation of aromatic organic hydroxy compound was achieved utilizing a divalent or trivalent manganese salt or cobalt (II) salts and organic cocatalysts such as hydroquinone or benzoquinone in combination with a palladium catalyst. Although the aforementioned cocatalyst system provided improved yields of aromatic organic carbonate as a result of the carbonylation of aromatic organic hydroxy compounds, using inorganic cocatalyst, such as cobalt (II) salts, methods for achieving higher rates of aromatic organic carbonate production are constantly being evaluated. In addition, it is also known that a variety of organic cocatalysts can sometimes degrade after being used at elevated pressures and temperatures in a carbonylation reaction and then exposed to ambient conditions of pressure and temperature. Color bodies also can be formed in the reaction mixture.